The present invention, in some embodiments thereof, relates to photovoltaic solar cells and to methods of manufacturing such cells. More particularly, but not exclusively, it is directed to methods of manufacturing metal contacts at the front and back surfaces of solar cells.
Photovoltaic cells are cells that absorb light photons (such as from sunlight) and convert the light photons into electricity. The cells generally comprise silicon substrates and the absorbed photons are converted into electron-hole pairs. The substrates generally include contacts to n-type silicon for collection of the electrons and contacts to p-type silicon for collection of the holes. The silicon substrates may typically be of a p or n-type. For purpose of explanation of the present invention, reference will be made to a p-type silicon substrate, with an n-type layer to form a p-n junction. However, it is noted that the same explanations, with the necessary changes, generally apply to an n-type substrate with a p-type layer.
In the art, there are several layouts of photovoltaic cells. One widely used layout for a p-type substrate has the p-n junction at the front surface of the substrate. It has contacts to the n-layer of the p-n junction at the front surface of the substrate and contacts to the bulk of the p-type substrate at the rear surface of the substrate.
An n-type doped layer is first produced on the front surface of the p-type substrate. The front side of the substrate is usually covered with a passivation layer. In order to provide a contact to the n-type layer at the front side, an opening in the passivation layer is formed into which metal is deposited (as for example by screen printing) such that the metal contacts the n-type layer. The contacts to the n-type layer form a grid or fingers on the front surface of the substrate. Alternatively, a paste of metal and glass frit is laid on the passivation layer. When the substrate is heated, the metal penetrates through the passivation layer and contacts the n-layer.
The rear surface of the substrate generally includes contacts to the bulk of the substrate and is also used for back reflecting the photons that were not converted into electrons-hole pairs during their propagation to the rear surface. In addition, adding a passivation layer to the rear surface increases the effectiveness of the cell.
In the prior art, the contacts to the p-type silicon at the rear surface of the substrate are formed either as a grid or as a metal layer coating the entire rear surface. Forming a grid on the rear surface, as opposed to full metal coating, could reduce the back reflection.
The prior art methods that form a full metal coating on the entire rear side, typically do not form a passivation layer on the rear side. One of the reasons for not providing passivation is the combination of the fact that the full aluminum coating provides moderate passivation and addition of passivation layer has manufacturing limitations.
The prior art described some methods for producing contacts at the rear surface including a passivation layer. GLUNZ, S. W. et al. “Laser-Fired Contact Silicon Solar Cells on p- and n-Substrates”, 19th European Photovoltaic Solar Energy Conference, June 2004, France describes forming a full Al coating on top of a passivation layer and then laser firing the full Al coatings at specific points thereof through the passivation layer. Firing causes the fired points to alloy with the passivation layer and contact the bulk of the substrate. The reflectivity in this method is achieved by the full metal coating at the rear surface.
Another method known in the art is creating openings in the passivation layer and depositing metal therein, usually by screen printing. Some of the prior art designs produce a p+ type layer in the opening to which the contact will connect. In general, the p+ layer is achieved by boron diffusion which creates a p+ type layer at an entire surface of the opening. A known design of such a cell is named PERF cells, as described in GREEN, M. “Crystalline Silicon solar Cells”, Clean Electricity From Photovoltaics, 2001, Chapter 4. The PERF Cells are produced by photolithography which requires an accurate registration between the openings in the passivation layer and doped layer and the metal deposited therein. The PERF cells provide an n+ layer on the rear side which improves its passivation properties.
The rear side passivation is generally produced before forming the metal grid. The prior art uses this order of actions since the production of the metal grid cause residues on the substrate at the area of connection which have to be cleaned before producing a passivation layer. As this is known to be a complex procedure, the passivation layer is generally produced before the contacts.
Some of the passivation layers used in the art do not form good passivation when applied on a p-type substrate and therefore the prior art uses expensive passivation materials which are produced with a generally complex procedure. Some of the prior art methods add a generally floating n-type doped layer at the rear surface between the bulk of the substrate and the passivation layers. Other methods use a passivation layer that creates an induced emitter at the rear surface. However, having a doped layer or induced emitter at the rear surface could cause shunting between the contacts and the doped layer or induced emitter. Therefore the contacts to the bulk of the substrate should preferably not contact the doped layer at the rear side. In avoiding this problem these designs are considered more complicated to produce and therefore also more expensive.
Another layout of photovoltaic substrates provides both contacts to n-type and p-type silicon areas produced at the rear surface of the substrate. In this layout there is a p-n junction formed at the rear surface and no connection and generally no p-n junction is formed on the front surface.
U.S. Pat. No. 7,339,110, for example, describes a manufacturing method of n-type substrates with both contacts to p- and n-type silicon at the rear side of the substrate. The method includes about twenty steps, three of which are patterning steps requiring accurate alignment. The complex manufacturing process substantially increases the cost of manufacturing such cells.
As yet unpublished PCT Application no. PCT/IL2009/000608 filed on Jun. 18, 2009 teaches a method of producing contacts at the front and/or rear sides of the substrate where the openings in the passivation layer and the deposition of metal are performed by a same laser or two different lasers with the same optical set-up. The use of the same laser or set-up enables automatic and substantially exact alignment of the openings and the deposited metal such that a relative thick layer of metal can be deposited with one laser operation. The methods of producing contacts disclosed in this application are applicable to some embodiments of the present invention and are included herein by reference to the extent of their applicability.
Additional background art includes HOFFMAN M. et al., “PECVD-ONO: A New Deposited Firing Stable Rear Surface Passivation Layer System for Crystalline Silicon Solar Cells”, Hindawi Publishing Corporation, Advances in OptoElectronics, Vol. 2008, Article ID 485467; GROHE, A., “Laser Technology for Contacting High-Efficiency Silicon Solar Cells, The Laser Fired Contacts Approach”; Agostinelli, G. et al. “Local Contact Structures for Industrial PERC-Type Solar Cells”; CHOULAT, P. et al, “High efficiency industrial type PERC Solar Cell on very thin EFG Substrates”; Agostinelli G. et al. “Silicon Solar Cells on Ultra-Thin Substrates for Large Scale Production”, Presented at the 21st EU PVSEC, Dresden, 2006, Choulat P. et al. “Above 17% industrial type PERC Solar Cell on thin Multi-Crystalline Silicon Substrate”, 22nd European Photovoltaic Solar Energy Conference and Exhibition, 3-7 Sep. 2007 Milano, Italy; ROMIJN, I. et al., “Aspire: A New Industrial MWT Cell Technology Enabling High Efficiencies on Thin and Large MC-Si Wafers” and CESAR, I. et al. “Benchmark of Open Rear Side Solar Cell with Improved Al-BSF Process at ECN”, Presented at the 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, 1-5 Sep. 2008.